Power supply and image forming apparatus

ABSTRACT

A power supply includes a first switching unit, a power restriction unit connected between the first switching unit and a load, a second switching unit connected between the power restriction unit and the load, a controller configured to output a control signal to the first switching unit and the second switching unit, and an adjusting unit configured to adjust input of the control signal to the second switching unit, and the second switching unit is operated selectively in accordance with the control signal.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/157,259, filed May 17, 2016, entitled “POWER SUPPLY ANDIMAGE FORMING APPARATUS”, the content of which is expressly incorporatedby reference herein in its entirety. Further, the present applicationclaims priority from Japanese Patent Application No. 2015-110371, May29, 2015, which is also hereby incorporated by reference herein in itsentirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a power supply, and particularly to astep-down power supply capable of changing an output voltage in a widerange and supplying power to a load.

Description of the Related Art

In general, a fan used in an electronic device has, in terms of noise,the function of changing the rotation speed thereof in accordance withan operation mode of the device. Control is performed such that therotation speed of the fan is increased and cooling of the inside of thedevice is prioritized when the device is being operated, and therotation speed of the fan is reduced to reduce noise when the device ison standby. In general, the rotation speed of a fan changes inaccordance with a supplied voltage. Thus, as a power supply that drivesa fan, a step-down power supply is used that is capable of changing anoutput voltage in a wide range (see Japanese Patent Laid-Open No.2007-116804).

In the case where an output voltage is not changed, a power supplycircuit in Japanese Patent Laid-Open No. 2007-116804 performs aswitching operation in a frequency range in which certain power supplyefficiency is achieved by feedback control. In contrast, in the casewhere the output voltage is changed, a switching operation is performedusing a switching-frequency control table based on setting voltages.Here, it is known that in the case where the output voltage is changedin a wide range with a less expensive configuration and where a loaddoes not require high voltage accuracy, control is performed using aswitching control table based on setting voltages, without performingfeedback control.

In the following, a conventional step-down power supply circuitdescribed above will be described with reference to FIG. 5A. This powersupply circuit is comprised of a switch circuit and an output circuit.The switch circuit switches on and off switching elements Q1 and Q2using an FPWM signal. A direct input voltage Vin input to an inputterminal is then converted into a periodic pulsed-wave signal Vpulse.This signal is smoothed by an output circuit comprised of a diode D2, aninductor L1, and a capacitor C1, and is output to an output terminal asan output voltage Vout. In this circuit, without performing feedbackcontrol, an output voltage is changed by changing the pulse width of theFPWM signal.

Here, a circuit as illustrated in FIG. 5B and in which a resistor R3 isused instead of the diode D2 and the inductor L1 in the output circuithas conventionally been used. In this circuit, the periodic pulsed-wavesignal Vpulse output from the switch circuit is smoothed by the resistorR3 and a capacitor C1, and is output as an output voltage Vout.Similarly to as in the circuit of FIG. 5A, an output voltage is alsoable to be changed by changing the pulse width of an FPWM signal in thiscircuit.

In addition, in the case where an output voltage is adjusted to aconstant value and where a load requires high voltage accuracy, acircuit as illustrated in FIG. 5C has conventionally been used. In thiscircuit, the output voltage is able to be switched between two types ofvoltage: a voltage close to the level of the input voltage (hereinafterreferred to as input-voltage level), and a voltage lower than theinput-voltage level. In the case where the voltage close to theinput-voltage level is output, the voltage is output by setting an F_ONsignal to High and switching on switching elements Q3 and Q4. In thecase where the voltage lower than the input-voltage level is output, thevoltage is output with high voltage accuracy by setting an F_H_ON signalto High, switching on switching elements Q1 and Q2, and conducting aZener diode ZD2. Regarding paths for these two output voltages, acontrol signal and a control signal line are necessary for each of thepaths in this circuit.

However, the inductor L1 used in the conventional art illustrated inFIG. 5A is arranged on a route through which power is supplied to aload, and thus a large part needs to be used, thereby increasing thecircuit in size. In contrast, in the conventional art illustrated inFIG. 5B and that does not use the inductor L1, it is difficult to outputa desired voltage because of a voltage drop in the resistor R3 in thecase where the output voltage close to the input-voltage level isrequested. It is possible to output a voltage close to the input-voltagelevel by increasing the number of control signal lines and the number ofdedicated output lines as in the circuit illustrated in FIG. 5C;however, there is a problem in that when the number of output controlsignals is increased, the number of necessary CPU pins is increased.

SUMMARY OF THE INVENTION

The present invention makes it possible to provide, without increasingthe number of control signals, a power supply capable of outputting avoltage in a wide range with a simple configuration.

The present invention provides a power supply including a firstswitching unit configured to perform switching for an input voltage, apower restriction unit connected between the first switching unit and aload, a second switching unit connected between the power restrictionunit and the load and configured to perform switching for the inputvoltage, a controller configured to output a control signal to the firstswitching unit and the second switching unit, and an adjusting unitconfigured to adjust input of the control signal to the second switchingunit, and the controller operates the second switching unit selectivelyin accordance with the control signal.

In addition the present invention provides an image forming apparatusincluding an image forming unit configured to form an image, and a powersupply configured to supply power for forming an image, the image beingformed by the image forming unit, the power supply including a firstswitching unit configured to perform switching for an input voltage, apower restriction unit connected between the first switching unit and aload, a second switching unit connected between the power restrictionunit and the load and configured to perform switching for the inputvoltage, a controller configured to output a control signal to the firstswitching unit and the second switching unit, and an adjusting unitconfigured to adjust input of the control signal to the second switchingunit, and the controller operates the second switching unit selectivelyin accordance with the control signal.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a step-down power supply according to afirst embodiment.

FIG. 2 is a diagram illustrating an output voltage obtained when thepulse width of a pulse width modulation signal is changed in the powersupply of the first embodiment.

FIG. 3 is a circuit diagram of a step-down power supply according to asecond embodiment.

FIG. 4 is a diagram illustrating an output voltage obtained when thevoltage level of a voltage level signal is changed in the power supplyof the second embodiment.

FIGS. 5A to 5C are circuit diagrams of conventional power supplies.

FIGS. 6A and 6B are diagrams illustrating an application example of apower supply of the present invention.

DESCRIPTION OF THE EMBODIMENTS First Embodiment

FIG. 1 is a circuit diagram of a step-down power supply device accordingto a first embodiment. Note that elements the same as those of theconventional art will be denoted by the same reference numerals, anddescription thereof will be omitted. In FIG. 1, a pulse width modulationsignal FPWM (hereinafter referred to as FPWM signal) from a centralprocessing unit (CPU) 100 serving as an output-control-signal generationunit is output to a control terminal of a switching element Q1 and aresistor R2. The resistor R2 is connected between the CPU 100 and theanode terminal of a diode D1, and a capacitor C2 is connected betweenthe resistor R2 and GND and between the anode terminal of the diode D1and GND. The FPWM signal output from the CPU 100 is converted by theresistor R2 and the capacitor C2 into a direct-current voltagecorresponding to the pulse width, and a control signal is generated forswitching on and off a second switch circuit. The anode terminal of thediode D1 is connected to the resistor R2 and the capacitor C2, and thecathode terminal of the diode D1 is connected to a control terminal of aswitching element Q4. This is provided to adjust a voltage at which thesecond switch circuit is switched on using a forward direction voltageof the diode D1. Note that transistors are used as the switchingelements Q1, Q2, and Q4 and a switching element Q3 in the presentembodiment. Here, in the present embodiment, the switching elements Q3and Q4 serving as a second switching unit are selectively operated inaccordance with the FPWM signal. Details of an operation will bedescribed in the following.

In FIG. 1, the CPU 100 stores, in an internal read-only memory (ROM)(not illustrated), a control table in which ON periods and OFF periodsof the FPWM signal corresponding to setting voltages are stored. It isthen possible to change an ON time period of the FPWM signal inaccordance with data regarding the ON periods and OFF periods stored inthe control table. In the case where a voltage lower than an inputvoltage is to be output, the CPU outputs an output control signal FPWMhaving a small pulse width (a short ON time period). Here, this outputcontrol signal FPWM is input to and smoothed by the resistor R2 and thecapacitor C2, and the resulting voltage is lower than the forwarddirection voltage of the diode D1. The switching elements Q3 and Q4 ofthe second switch circuit are thus not switched on. That is, the diodeD1 serves as a unit that adjusts the operation of the second switchcircuit in accordance with the output control signal FPWM from the CPU100. In contrast, the switching elements Q1 and Q2 of a first switchcircuit serving as a first switching unit, to which the output controlsignal FPWM is input, are switched on and off repeatedly. As a result,an input voltage Vin is converted into a periodic pulsed-wave signalVpulse. This signal Vpulse is smoothed by a resistor R3 and a capacitorC1, and is output as an output voltage Vout (a first voltage).

In the case where a voltage close to the input voltage (a second voltagealmost equal to the input voltage) is to be output, the CPU 100 outputsan output control signal FPWM having a large pulse width (a long ON timeperiod). Here, this output control signal FPWM is input to and smoothedby the resistor R2 and the capacitor C2, and the resulting voltage ishigher than the forward direction voltage of the diode D1. The switchingelements Q3 and Q4 of the second switch circuit are thus switched on. Incontrast, the switching elements Q1 and Q2 of the first switch circuit,to which the output control signal FPWM is input, are switched on andoff; however, a current does not flow through the switching element Q2of the first switch circuit. As a result, the voltage close to the inputvoltage is output as the output voltage Vout from the switching elementQ3 of the second switch circuit. This is because the resistor R3, apower restriction element, is provided on the power supply route of thefirst switch circuit, and the power supply route of the second switchcircuit has a smaller impedance.

FIG. 2 is a diagram illustrating an output voltage obtained when thepulse width of the FPWM signal is changed in the circuit of the powersupply device of the first embodiment. The horizontal axis representsthe percentage of the ON time period of the FPWM signal (hereinafteralso referred to as ON Duty), and the vertical axis represents theoutput voltage Vout. This shows that the output voltage Vout increasesas the ON Duty of the FPWM signal increases. The switching elements Q3and Q4 of the second switch circuit start switching on and off at thepoint in time when the ON Duty exceeds 55%, the switching element Q3 andQ4 of the second switch circuit enter an ON state at the point in timewhen the ON Duty exceeds 65%, and the voltage close to the input voltageis output, the input voltage being 24 V.

For example, a fan that cools the inside of an apparatus is able to beapplied as a load 200 in FIG. 1. In the case where it is desired toreduce the sound of the fan, the ON Duty of the FPWM signal is set to20%, and approximately a voltage of 11 V is supplied to the fan, therebyreducing the rotation speed of the fan. In addition, in the case wherethe cooling function of the fan is prioritized, the ON Duty of the FPWMsignal is set to 100%, and a voltage close to the input voltage, whichis 24 V, is supplied to the fan, thereby increasing the rotation speedof the fan.

According to the present embodiment, without increasing the number ofinput signals, the output voltage is able to be switched between certainvoltages with a simple circuit configuration. Specifically, the outputvoltage is able to be changed up to a voltage close to the input voltageby changing the pulse width of the FPWM signal. In addition, the voltageto be supplied to the load 200 is able to be changed minutely.

Second Embodiment

FIG. 3 is a circuit diagram of a step-down power supply device accordingto a second embodiment. The difference between the second embodiment andthe first embodiment is that the second embodiment is characterized inthat a voltage level signal F_ON (hereinafter referred to as F_ONsignal) is output as a signal output by a CPU 100, and the voltage ofthe F_ON signal is changed. As a result, the output voltage is able tobe switched between two types of voltage: a voltage close to an inputvoltage, and a voltage lower than the input voltage. In FIG. 3, from theCPU 100 serving as an output-control-signal generation unit, an F_ONsignal is output to a control terminal of a switching element Q1 and aresistor R5. The resistor R5 is connected between the CPU 100 and thecathode of a Zener diode ZD1, the cathode of the Zener diode ZD1 isconnected to the resistor R5, and the anode of the Zener diode ZD1 isconnected to a control terminal of a switching element Q4. This isprovided to adjust a voltage at which a second switch circuit isswitched on using a Zener diode breakdown voltage.

In the following, a circuit operation will be described. The CPU 100serving as the output-control-signal generation unit stores, in a ROM(not illustrated), a control table in which information indicatingvoltage levels corresponding to setting voltages is stored. Furthermore,the CPU 100 includes a conversion processing unit that reads outinformation indicating a voltage level from the control table andperforms D/A conversion. The CPU 100 outputs a result of D/A conversionas an output control signal F_ON.

In the case where a voltage lower than the input voltage is to beoutput, the CPU 100 outputs an F_ON signal smaller than the breakdownvoltage of the Zener diode ZD1. Here, the Zener diode ZD1 is notconducted, and a switching element Q3 and the switching element Q4 ofthe second switch circuit are not switched on. That is, the Zener diodeZD1 serves as a unit that adjusts the operation of the second switchcircuit in accordance with an output control signal, which is the F_ONsignal, from the CPU 100. In contrast, the switching element Q1 and aswitching element Q2 of a first switch circuit, to which the F_ON signalis input, are switched on, a Zener diode ZD2 is conducted, and thereby avoltage with high voltage accuracy is output as an output voltage Vout.

In addition, in the case where a voltage close to an input-voltage levelis to be output, the CPU 100 outputs an F_ON signal larger than thebreakdown voltage of the Zener diode ZD1. Here, the Zener diode ZD1 isconducted, and the switching elements Q3 and Q4 of the second switchcircuit are switched on. In contrast, the switching elements Q1 and Q2of the first switch circuit, to which the F_ON signal is input, areswitched on; however, a current does not flow through the switchingelement Q2 of the first switch circuit. As a result, a voltage close tothe input-voltage level is output as the output voltage Vout from theswitching element Q3 of the second switch circuit. This is because theresistor R3, a power restriction element, is provided on the powersupply route of the first switch circuit, and the power supply route ofthe second switch circuit has a smaller impedance.

FIG. 4 is a diagram illustrating an output voltage obtained when thevoltage level of the F_ON signal is changed in the circuit of the powersupply device of the second embodiment. The horizontal axis representsthe voltage level of the F_ON signal, and the vertical axis representsthe output voltage Vout. This shows that the output voltage Voutincreases as the voltage level of the F_ON signal increases. Theswitching elements Q1 and Q2 of the first switch circuit are switched onat the point in time when the voltage level exceeds approximately 1 V,and approximately 12 V is output. In addition, the switching elements Q3and Q4 of the second switch circuit are switched on at the point in timewhen the voltage level exceeds approximately 2.6 V, and a voltage closeto the input voltage, which is 24 V, is output.

For example, a fan that cools the inside of an apparatus is able to beapplied as a load 200 in FIG. 3. The ON Duty of an FPWM signal is set to20%, and approximately a voltage of 11 V is supplied to the fan, therebyreducing the rotation speed of the fan. In addition, in the case wherethe cooling function of the fan is prioritized, the ON Duty of the FPWMsignal is set to 100%, and a voltage close to the input voltage, whichis 24 V, is supplied to the fan, thereby increasing the rotation speedof the fan.

In the case where it is desired to reduce the sound of the fan, thevoltage level of the F_ON signal is set to 1.8 V, and approximately avoltage of 12 V is supplied to the fan, thereby reducing the rotationspeed of the fan. In addition, in the case where the cooling function ofthe fan is prioritized, the voltage level of the F_ON signal is set to3.3 V, and a voltage close to the input voltage, which is 24 V, issupplied to the fan, thereby increasing the rotation speed of the fan.

According to the present embodiment, without increasing the number ofinput signals, the output voltage is able to be switched between certainvoltages with a simple circuit configuration. Specifically, the outputvoltage is able to be switched between two types of voltage by changingthe voltage of the F_ON signal, the two types of voltage including avoltage close to the input-voltage level and a voltage lower than theinput-voltage level.

Note that, configurations using transistors as the switching elements Q1to Q4 have been described in the above-described first and secondembodiments; however, not only transistors but also field-effecttransistors (also referred to as FETs) may be used.

(Application Example of Power Supply Device)

The power supply devices described in the above-described embodimentsare each applicable as, for example, a low-voltage power supply for animage forming apparatus and as a power supply that supplies power to adriving unit such as a motor. In the following, the configuration of animage forming apparatus to which the power supply devices described inthe above-described embodiments may be applied will be described.

[Configuration of Image Forming Apparatus]

As an example of an image forming apparatus, a laser beam printer willbe described as an example. FIGS. 6A and 6B illustrate a schematicconfiguration of a laser beam printer, which is an example of anelectrophotographic printer. A laser beam printer 500 includes aphotoconductive drum 511, a charge unit 517 (a charge unit), and adevelopment unit 512 (a development unit). The photoconductive drum 511serves as an image bearing member on which an electrostatic latent imageis formed. The charge unit 517 charges the photoconductive drum 511uniformly. The development unit 512 develops with toner theelectrostatic latent image formed on the photoconductive drum 511. Thetoner image developed on the photoconductive drum 511 is thentransferred by a transfer unit 518 (a transfer unit) onto a sheet (notillustrated) as a recording material supplied from a cassette 516, thetoner image transferred to the sheet is fixed by a fuser 514, and theresulting sheet is discharged to a tray 515. The photoconductive drum511, the charge unit 517, the development unit 512, and the transferunit 518 are an image forming unit. In addition, the laser beam printer500 includes a power supply device 550, which is described in theembodiments above. Note that an image forming apparatus to which powersupply devices 550 described in the first and second embodiments areapplicable is not limited to the one illustrated in FIGS. 6A and 6B, andmay also be, for example, an image forming apparatus including aplurality of image forming units. Furthermore, the image formingapparatus to which the power supply devices 550 are applicable may alsobe an image forming apparatus including a primary transfer unit thattransfers a toner image formed on the photoconductive drum 511 onto anintermediate transfer belt, and a secondary transfer unit that transfersthe toner image formed on the intermediate transfer belt onto a sheet.

The laser beam printer 500 includes a controller 520 that controls animage forming operation performed by the image forming unit and a sheetconveyance operation. Each of the power supply devices 550 described inthe above-described embodiments is able to supply power to, for example,a driving unit 551 such as a motor used to rotate the photoconductivedrum 511 or used to drive various types of roller and the like thatconvey sheets. In addition, when the power supply device 550 is a deviceincluding a cooling fan 552 for cooling heat-producing portions insidethe device, the rotation speed of the fan 552 is able to be changed asdescribed above.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

What is claimed is:
 1. A power supply for supplying power to a load, thepower supply comprising: a first switching unit configured to perform aswitching operation to convert an input voltage to an output voltage,switching on and off being repeatedly performed in the switchingoperation in accordance with a control signal; a power restriction unitconfigured to input the control signal; and a second switching unitconfigured to input the control via the power restriction unit, andconfigured to perform switching on and off selectively in accordancewith a control signal, wherein the second switching unit is in an offstate in a case where a voltage value according to the control signal issmaller than a threshold value and switches to an on state from the offstate in a case where a voltage value according to the control signalexceeds the threshold value.
 2. The power supply according to claim 1,wherein the control signal is a pulse signal, and wherein in a casewhere an on period of the pulse signal exceeds a predetermined value, avoltage input to the power restriction unit exceeds the threshold value.3. The power supply according to claim 2, further comprising, a controlunit configured to output the control signal, wherein the control unitstores data for changing an on period of the pulse signal.
 4. The powersupply according to claim 1, wherein the power restriction unit is adiode, and wherein the threshold value is a forward direction voltage ofthe diode.
 5. The power supply according to claim 1, further comprisinga circuit configured to convert a voltage to a direct current voltage,wherein the circuit is connected to the power restriction unit on a sidefrom which the control signal is input.
 6. The power supply according toclaim 1, wherein the output voltage to be supplied to the load when thesecond switching unit is off state is smaller than the output voltage tobe supplied to the load when the second switching unit is on state. 7.The power supply according to claim 1, wherein the load is a fan thatrotates, and wherein a number of rotations of the fan in a case wherethe second switching unit is in the off state is less than a number ofrotations of the fan in a case where the second switching unit is in theon state.
 8. An image forming apparatus including an image forming unitfor forming an image, the image forming apparatus comprising: a powersupply configured to supply power to a load that operates in a casewhere an image is formed, wherein the power supply includes a firstswitching unit configured to perform a switching operation to convertthe input voltage, switching on and off being repeatedly performed inthe switching operation in accordance with a control signal; a powerrestriction unit configured to be input the control signal; and a secondswitching unit configured to be input the control via the powerrestriction unit, and configured to perform switching on and offselectively in accordance with a control signal; wherein the secondswitching unit is in an off state in a case where a voltage valueaccording to the control signal is smaller than a threshold value andswitches to an on state from the off state in a case where a voltagevalue according to the control signal exceeds the threshold value. 9.The image forming apparatus according to claim 8, wherein the controlsignal is a pulse signal, and wherein in a case where an on period ofthe pulse signal exceeds a predetermined value, a voltage input to thepower restriction unit exceeds the threshold value.
 10. The imageforming apparatus according to claim 9, further comprising a controlunit configured to output the control signal, wherein the control unitstores data for changing an on period of the pulse signal.
 11. The imageforming apparatus according to claim 10, wherein the power restrictionunit is a diode, and wherein the threshold value is a forward directionvoltage of the diode.
 12. The image forming apparatus according to claim8, further comprising a circuit configured to convert a voltage to adirect current voltage, wherein the circuit is connected to the powerrestriction unit on a side from which the control signal is input. 13.The image forming apparatus according to claim 8, wherein the outputvoltage to be supplied to the load when the second switching unit is offstate is smaller than the output voltage to be supplied to the load whenthe second switching unit is on state.
 14. The image forming apparatusaccording to claim 8, wherein the load is a fan that rotates for coolingan inside of the image forming apparatus, and wherein a number ofrotations of the fan in a case where the second switching unit is in theoff state is less than a number of rotations of the fan in a case wherethe second switching unit is in the on state.
 15. The image formingapparatus according to claim 8, further comprising a control unitconfigured to control an operation of the image forming apparatus,wherein the control unit outputs the control signal.